Gas springs are well known and have been used in dies of presses for sheet metal stamping operations. Conventional gas springs have a gas chamber which receives a pressurized gas that provides a force on a piston and a piston rod of the gas spring to bias them to an extended position. The pressurized gas resists the movement of the piston and the piston rod from their extended position to a retracted position. Various retainers and seals are provided in the gas spring to retain the piston and piston rod within a casing of the gas spring and to prevent leakage of the pressurized gas from the gas chamber.
Typically, a plurality of gas springs yieldably urge a binder ring or clamp ring of the die assembly into engagement with a sheet metal work piece as the dies are closed by the press to cold form the work piece. In presses with mechanical drive mechanisms, the mechanical advantage of the mechanism varies depending on the position of a ram of the press, with the mechanical advantage typically increasing dramatically as the ram approaches a fully advanced or extended position to completely close the dies and form the work piece.
When the ram of the press is in its mid-stroke position, the piston and rod of the gas spring are typically in their fully extended position. The piston and rod are initially moved toward their retracted position well before the ram reaches it fully extended position. Consequently, the gas springs initially apply a relatively large force or load to the drive mechanism of the press relative to the force the press can then produce. In addition, since the mechanical drive mechanism of the press has already begun moving and accelerating the ram and the die half attached thereto before it begins to actuate and overcome the resistance produced by the gas springs, the initial impact or actuation of the gas springs creates an impulse or load spike of relatively high magnitude and short duration on the drive mechanism of the press. The load spikes can cause an increase in the necessary press maintenance, thereby increasing the maintenance and repair costs of the press. In some instances, the spike loads may seriously damage the drive mechanism of the press, thereby decreasing the useful life of the press.
Similarly, during opening of the closed dies after a work piece has been formed, the gas springs apply a relatively high force or load to the drive mechanism of the press, which force is suddenly terminated when the piston rods become fully extended. As such, the drive mechanism is subjected to further impulse or load spikes. As mentioned above, the load spikes can result in damage to the press, for example, by causing vibration, or by causing the binder ring to bounce relative to mating components.